1. Field of the Invention
The present invention relates to an image forming apparatus that performs image formation with an electrophotographic system, such as a copier, laser printer, facsimile, etc.
2. Description of Related Art
An image forming apparatus such as a copier includes a photoconductor drum, a charging device for uniformly charging the photoconductor drum, an exposure device for exposing the photoconductor drum so as to form a latent image, and a developing device for developing the latent image. Characteristic change appears in these components and developer stored in the developing device due to environmental change such as temperature or humidity, or due to time-course change. The characteristic change changes the formation state of the image obtained by charging the photoconductor drum, exposing the drum, and developing a latent image. There has been known a technique in which a process, called a process control, for changing an image forming condition at a predetermined timing is executed in order to reduce the change of the formation state of the image (refer to Patent Documents 1 and 2).
In the process control, in general, an image of a test pattern is actually formed, the density of the formed image is measured, and an image forming condition is changed for eliminating the difference between the measured value and the ideal value. Therefore, when the process control is executed, toner is somewhat consumed. During the execution of the process control, a user who cannot use the image forming apparatus may have to suffer inconvenience.
When timing for executing the process control is too late, the image formation state becomes unstable, while when the timing for executing the process control is too early, toner is wastefully consumed, or job efficiency is wastefully reduced. Accordingly, it is important that the process control is executed at appropriate timing.
In the Patent Document 1: Japanese Unexamined Patent Application No. 2003-91224, the timing for executing the process control is made appropriate by determining the timing for executing the process control on the basis of the ratio of the number of monochrome printing sheets and the number of color printing sheets.
In the Patent Document 2: Japanese Unexamined Patent Application No. 2005-352379, the apparatus recognizes that a printing with an printing area ratio of 3 to 30% is a text printing, and the number of printing sheets for which the process control is executed is switched between the text printing and color printing, whereby the timing for executing the process control is made appropriate.
To what degree the deterioration of a printing quality is allowed is generally different from each user. In the techniques in the Patent Documents 1 and 2, timing of the process control optimum for each user cannot be set.